Secondary power systems of the type used in single engine military aircraft present significant and unique challenges to designers. Such power systems are typically required to provide a highly reliable and virtually uninterrupted source of power to flight or mission critical accessories or subsystems on the aircraft despite exposure to extremes in temperature and altitude, and after sustaining battle related damage to a portion of the power system. Such systems must also be easy to repair and maintain in remote areas and under the time pressures incident with the all weather, day and night, character of modern warfare.
In the jargon of aircraft power systems, the term "Primary Power System" is generally meant to include only the primary propulsion engine, and the term "Secondary Power System" is sometimes used in a broad sense to include all power consuming accessories, gearboxes, accessory drives, and power sources on the aircraft other than the propulsion engine. The term "Secondary Power System" is used herein in a somewhat narrower context intended to include only those accessories, gearboxes, accessory drives, and secondary power sources receiving rotating shaft power from the propulsion engine, or delivering rotating shaft power to the propulsion engine.
Specifically, a typical secondary power system of the type contemplated by our invention includes one or more accessories requiring a rotational drive input, such as electrical generators, hydraulic pumps, or environmental control equipment providing air pressurization and conditioning. A Secondary Power System according to our invention may also include one or more secondary power sources, often known as Auxiliary Power Units (APU's) or Emergency Power Units (EPU's), that can be selectively coupled to drive a portion of the secondary power system, while the aircraft is on the ground with the propulsion engine not operating, or in flight after a failure of the propulsion engine or a portion of the drive system.
Virtually all aircraft secondary power systems include some form of engine gearbox (EGB) operably connected to receive rotating shaft power from the propulsion engine, and most are configured to provide multiple mechanical drive shafts for connection to the accessories. Engine gearboxes also typically include gear trains to convert engine RPM into the proper speed for the accessories driven by those drive shafts.
The secondary power system in multi-engine commercial aircraft has conventionally utilized an engine gearbox (EGB) on each engine, with each EGB including appropriate drive accommodations for all accessories mechanically connected to the engine by drive shafts. Typical accessories include an electrical generator, hydraulic pumps, an air turbine starter for the propulsion engine, and engine driven fuel pumps. For this type of installation, there is generally a common oil circuit provided for lubrication of the engine and all of the accessories mounted on the EGB. Other functions such as cabin pressurization and cooling, pneumatic power, and auxiliary or emergency power from an APU and a Ram Air Turbine (RAT) are typically coupled to the engine via pneumatic, hydraulic, or electrical lines and are thus outside the scope of the definition of secondary power system adopted herein because these devices are not directly mechanically driven by or connected to the EGB.
The guiding rationale for selecting a single EGB arrangement and the shared oil system in such aircraft may be that, because there are multiple propulsion engines each having a separate Secondary Power System, and because there is also available APU and or EPU equipment that can be coupled to the propulsion engine without use of the Secondary Power System in the narrower definition contemplated by this invention, there is sufficient redundancy on the aircraft so that if a given component in the Secondary Power System on one engine fails, that component or the secondary power system that it is a part of can be simply decoupled and shut down without adversely affecting overall aircraft operations or risking contamination of the shared oil circuit as a result of the failure of an accessory.
In single engine military planes, however, a different approach is generally taken. Here the engine typically includes an engine mounted gearbox which drives only those accessories dedicated to the engine itself, such as engine driven fuel pumps, a small generator providing electrical power for ignition and control of the engine itself, and a hydraulic pump to provide lubrication and cooling of the engine bearings. Other accessories are typically mounted on a separate gearbox connected to and driven by an output shaft of the EGB. In some instances this separate gearbox is mounted directly on the EGB and is known as an Engine Mounted Accessory Drive (EMAD). In other cases, the separate gearbox is mounted on the airframe and connected to the EGB shaft by a separate drive shaft that allows tolerance for dimensional differences and relative movement between the EGB and the separate gearbox. Where the separate gearbox is mounted on the airframe rather than the EGB, it is known as an Aircraft Mounted Accessory Drive (AMAD). In both an EMAD and AMAD based system, a separate lube oil system for the EGB lubrication system is typically provided for the EMAD or AMAD. It is also common in military aircraft to have the APU or EPU coupled mechanically directly to the EMAD or AMAD, if they are present.
The inclusion of either an AMAD or an EMAD in the secondary power system of a military aircraft to allow isolation of those accessories dedicated to the engine does not come without a price. Adding the AMAD or EMAD increases the complexity, cost, size and weight of the secondary power system, and because there are now more parts to potentially fail, also decreases reliability. These factors have driven the designers of prior Secondary Power Systems to provide only a single EMAD, or alternatively a single AMAD in addition to the EGB.
One major drawback of prior EMAD based systems is that because the EMAD is mounted on the EGB, it is typically necessary to remove the EMAD and all of its accessories with the engine, if engine replacement is required. Such a removal typically necessitates draining and disconnecting hydraulic lines extending from pumps mounted on the EMAD. This is a difficult and time consuming task.
The use of an AMAD rather than an EMAD theoretically alleviates some of the problems related above with regard to the difficulty of engine removal. In some installations however, there is not enough room adjacent the EGB, or there are other physical constraints that necessitate the use of an EMAD rather than an AMAD. Even if an AMAD is used rather than an EMAD, removal of the AMAD prior to removal of the engine might have still be required if the AMAD was mounted in a location hampering removal of the engine.
The overall approach taken with regard to designing secondary power systems for prior single engine military aircraft evinces a strong desire to keep the lube oil circuits for the engine and EMAD or AMAD separate to minimize the risk of contaminating the engine lubrication circuit or EGB in the event of a failure in one of the accessories. The prior approach with regard to acceptability of mounting the EMAD in a manner requiring its removal with the engine evinces a bias toward keeping the secondary power system as compact and tightly packaged about the engine as possible to minimize the overall size and radar signature of the aircraft, even if that meant making engine replacement more difficult. Also it may have been felt that the accessories were so much less reliable than the propulsion engine that they should be more readily removable than the engine, or that the additional parts required to allow removal of the engine without disturbing the EMAD imposed too great a liability in terms of increased complexity, cost, size or weight, and unacceptable reductions in the reliability of the Secondary Power System.
Although the prior approaches outlined above were acceptable in their day and were no doubt derived through assumptions that were valid at the time, further improvements are both possible and necessary for the next generation of single engine aircraft. For instance, although it may have been acceptable in the past to mount an AMAD or an EMAD in a manner that required removal of the AMAD or EMAD and its accessories to replace an engine, such an approach is not acceptable for the next generation of single engine military aircraft. Furthermore, with the passage of time the reliability of the accessories and secondary power sources included in the secondary power system have improved to the point that old biases are no longer valid. Continuing to use the old assumptions and biases imposes undesirable and unnecessary constrains on the design and operational flexibility of the next generation of secondary power systems and the aircraft carrying them.
In a new single engine military aircraft currently being designed, it is highly desirable during both initial assembly and subsequent repair operations to be able to remove and replace the main engine without removal of the EMAD or the AMAD, or any of the accessories mounted thereon. It is also highly desirable to not have to break any of the hydraulic or pneumatic lines, fuel lines, or electrical connections on any of the accessories mounted on the EMAD or AMAD, whichever the case may be, incident removal of the main engine.
With the faster paced, all weather, day or night, character of modern warfare, it is now more important than ever to be able to get a military aircraft back into service as quickly as possible, even after a major repair such as removal and replacement of a main engine. It is also highly desirable to take advantage of new higher reliability accessories and new system configurations to increase the flexibility of partial operation of the secondary power system after a partial failure of that system, and thereby enhance the overall operational capability of the aircraft.
It is an object of our invention, therefore, to provide an improved single engine aircraft. Further objects of our invention include providing:
1) an improved secondary power system for an aircraft; PA1 2) a secondary power system with improved partial operational flexibility after a failure of an accessory or a primary or secondary power source; and PA1 3) a secondary power system with improved maintainability.